Big screen micro display based rear projection imaging systems, such as televisions and monitors are becoming increasingly popular with consumers. This is due to their combination of low weight, high picture quality, price, and reliability over comparable large display technologies such as cathode ray tube (CRT) based rear projection imaging systems, plasma displays, and direct view liquid crystal displays (LCD).
Micro display based rear projection imaging systems generally operate by shining a light on or through single or multiple micro displays (also known as imagers or light-valves) where an image is displayed. Micro display imagers, such as digital micro mirror devices (DMD) imagers, liquid crystal on silicon (LCoS) imagers, and LCD imagers, range from one half to one inch along their diagonal. The light that shines through or is reflected from a micro display forms a picture and is directed by projection optics onto the back surface of a transmissive display screen, thereby showing the picture from the micro display. In the process of displaying the picture on the screen the picture may be magnified 50 to 150 times, depending on the size of the micro display imager and the size of the screen.
Consumers are particularly interested in shallow rear projection monitors for aesthetic and space saving reasons. Reasonable manufacturing tolerances, cost, and optical limits, however, restrict the depth a rear projection monitor may achieve. One such optical limit is due to the high image magnification necessary to project images and the short distance, generally less than two feet, available to perform the magnification. In order to perform this level of magnification in a short light path distance, the use of very wide-angle optics is required. These lenses are expensive and complicated.
One solution to increasing the optical light path distance in a desired monitor cabinet depth is to use one or more mirrors to reflect the image from the micro display projector on to the screen. Adding a mirror or mirrors to an imaging system causes the optical light path to fold, increasing its effective length within a given front-to-back distance or depth.
Conventional rear projection imaging systems solutions generally utilize either one or two flat mirrors. In the single mirror case, the micro display projector points away from the rear projection monitor screen and shines on a mirror. The mirror then reflects the micro display projector's light rays on to the monitor screen. In the double mirror case, the micro display projector shines, sometimes in a direction parallel to the monitor screen, to a small first mirror. This first mirror then reflects the micro display projector's light rays on to a large second mirror, which directs the light rays to the monitor's screen. The second mirror is usually substantially parallel to the monitor's screen.
Image quality is an important consideration when a consumer purchases a rear projection imaging system. Consumers do not want to buy rear projection imaging systems that display warped, distorted, dim, or discolored images. Designers therefore spend a significant amount of time optimizing image quality given other design constraints, such as cabinet depth, system cost, and others. Flat perfect mirrors do not affect the optical properties of a system, other than allowing clever management of the optical light path in a system that has limited external dimensions. Therefore, most optical parameter considerations are addressed in the design of the optics on the micro display projector. Even in rear projection imaging systems using mirrors to lengthen the optical light path distance, these lenses are still very expensive to both design and manufacture, sometimes containing up to twenty-six elements.
Designers can, however, decrease the cost and complexity of the projection optics in the projector lenses by replacing the second mirror with an additional optical element. If the second mirror is replaced with a large curved mirror, the curved mirror will Form part of the optical projection system. The specific shape of the large curved mirror can be changed and optimized in conjunction with the projection lens to achieve a system which has lower overall complexity and cost in the refractive projection lens. Geometrical distortions and lateral color effects in the image can be taken care of in hardware and/or software through introducing compensating distortions in the video image as it passes through the electronics to the imaging device.
Designing the optical elements in a rear projection imaging system is usually done in two stages with the help of a software package such as Zemax, or Code V. The first stage is a rough design stage where optical components are selected and positioned within the system, perhaps using paraxial lens models, and the second stage is an optimization stage where components, exact optical materials and surface properties, distances between optical components, and the like are fine-tuned to optimize around a set of parameters. Lens optical elements are adjusted and the curved mirror's surface is varied in this stage. It is nearly impossible to produce a completely optimized imaging system, so the optimization stage relies on weighting specific parameters. These parameters may include imaging focus, the geometry of the displayed image, chromatic aberration, and the like, and are generally represented by a merit function.
Designing a curved mirror that allows a software package to converge to a workable solution during the optimization stage is difficult. Common functions describing a curved mirror's surface, such as spherical and aspherical functions, do not necessarily have enough degrees of freedom for the software package to find a “real” solution. For example, a software package, given an initial condition of a spherical mirror or the like, may optimize the rear projection imaging system by replacing lens elements with elements that have negative thickness.
Designing and optimizing an optical mirror from first principles can be time consuming and can produce undesired results unless certain initial conditions are established before designing the mirror. Accordingly, a need exists for an optimized complex mirror surface to enable simpler and cheaper optics to be used in rear projection displays.